Magnetic Compass

• Deviation Curve
• Magnetic Variation

Usually a gyrocompass is not available at a yacht. Instead one or more magnetic compasses serve for steering. The reading of this kind of compass must be corrected for variation and deviation.
OpenCPN uses true course over ground. The course to steer is calculated taking into account the wind, the current and the compass correction: magnetic variation + magnetic deviation:

• Magnetic Variation - or magnetic declination is the angle between magnetic north and true north.
• Deviation curve of magnetic compass - Magnetic deviation is the error induced in a compass by local magnetic fields.